1. Field of Invention
The present invention relates to a process for making multi-crystalline silicon thin-film solar cells and, more particularly, to a high-temperature process for making multi-crystalline silicon thin-film solar cells based on plasma-enhanced chemical vapor deposition.
2. Related Prior Art
Silicon-based solar cells are generally made in low-temperature processes based on plasma-enhanced chemical vapor deposition (“PECVD”). An amorphous or microcrystalline silicon film is coated on a substrate of glass, aluminum, silicon, stainless steel or plastics. A back contact is made of aluminum, gold, silver or transparent conductive oxide such as indium-tin oxide (“ITO”) and zinc oxide.
The primary advantage of the low-temperature processes is the wide variety of materials that can be used to make the substrates. However, they suffer drawbacks such as defective silicon films, low photoelectrical conversion efficiencies and low light-soaking stability. In the PECVD, while coating the microcrystalline silicon film, a silicon material is highly diluted in hydrogen according to the following notion:[H2]/[SiH4]>15
That is, the concentration or flow rate of H2 is more than 15 times as high as that of SiH4. The problems with the PECVD include a low growth rate of the film, a long process and a high cost.
Regarding the making of the multi-crystalline silicon solar cells, there are various techniques such as solid phase crystallization (“SPC”) and aluminum-induced crystallization (“AIC”).
The SPC is based on the PECVD. In the SPC, an amorphous silicon film is deposited, intensively heated and annealed at a high temperature. Thus, a multi-crystalline silicon film with a grain size of 1 to 2 micrometers is made.
There are however problems with the low-temperature processes for making multi-crystalline silicon solar cells based on the PECVD. Firstly, many defects occur in the silicon films. Secondly, the photoelectrical conversion efficiencies are low. Thirdly, the light soaking stabilities are low. Fourthly, the growth rates of the films are low. Sixthly, the processes are long. Seventhly, the costs are high.
Referring to FIGS. 11 through 15, in the AIC, a substrate 71 is coated with an aluminum film 72. An amorphous silicon film 73 is coated on the aluminum film 72 based on the PECVD and annealed at a temperature of 575 degrees Celsius for a long time to form a seed film 74. Then, it is subjected to an epitaxial process such as the PECVD or an electron cyclotron resonance chemical deposition (“ECR-CVD”) to make a multi-crystalline silicon film 75. The AIC however involves many steps and takes a long time. The resultant grain size is 0.1 to 10 micrometers.
A conventional silicon-based tandem solar cell includes an upper laminate and a lower laminate. The upper laminate is an amorphous silicon p-i-n laminate. The lower laminate is a microcrystalline silicon p-i-n laminate. Thus, the infrared and visible light of the sunlit can be converted into electricity. However, the photoelectrical conversion efficiency of the conventional silicon-based tandem solar cell deteriorates quickly.
Concerning the process for making multi-crystalline silicon solar cells based on the AIC, the processes are long for including many steps and therefore expensive. As for the conventional silicon-based tandem solar cell, the photoelectrical conversion efficiency deteriorates quickly.
The present invention is therefore intended to obviate or at least alleviate the problems encountered in prior art.